This invention relates to reproduction apparatus, and more particularly, to an electrophotographic device having a removal or pick-off device for removing carrier beads from the developer mix used to develop latent images which adhere to a charge retentive surface in the apparatus during development.
In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Imaging systems using imagewise ion projection to a charge retentive surface to form an electrostatic latent image developable with toner operate similarly.
Developing material commonly used in systems for developing latent images on the charge retentive surface typically comprises a mixture of toner and a "carrier" of larger granular beads of a magnetic material. If the developing system is a magnetic brush assembly, magnetizable carrier beads also provide mechanical control for the formation of magnetic brush bristles so that toner can readily be brought into contact with the charge retentive surface. Toner is attracted to the latent image from the carrier beads to form the toner image.
It is not unusual for carrier beads to become highly charged in a development housing and adhere to the photoreceptor (p/r). This phenomenon, known as "Bead Carry-Out" (BCO) is a well known xerographic problem. BCO can create all sorts of undesirable effects. For example, beads that remain attached to an image area can cause transfer deletions that degrade copy quality. If, on the other hand, the carrier bead becomes detached during pre-transfer charging, it may fall into the corotron, or paper transport mechanism and lead to an electrical, or mechanical failure.
Normally, a carrier bead has the opposite charge polarity from it's companion toner and is repelled from the image area by the qE, where q is the toner charge and E is the electric field force. However, the qE force created by the cleaning field in the background regions will tend to move the charged carrier bead towards the photoreceptor. Fortunately, the qE force in the background regions is smaller compared to an image region. However, in unusual circumstances, the carrier bead can become charged to the same polarity as the toner. In this case, both the toner and the carrier are driven into the image area by the development field. This can happen when a carrier bead, heavily impacted with toner, becomes triboelectrically charged by other carrier; or a de-toned carrier bead, in momentary contact with a conductive part of the development housing, becomes inductively charged to the same polarity as the toner by the ambient electric fields within the housing.
It is more difficult for the development magnets, aided by gravity, to recapture a carrier bead once it loses its momentum, and becomes attached to the photoreceptor. The reason is the coulomb force between the carrier bead charge q.sub.c and the p/c charge q.sub.p, which is proportional to q.sub.c q.sub.p /r.sup.2, and the image force, which is proportional to q.sub.c.sup.2 /r.sup.2 both increase rapidly as the distance between the bead and p/r diminish. Small carrier beads (&lt;70 microns diameter), and flat, plate-like carrier beads, are particularly prone to BCO because the bead's charge is proportional to its surface area (.about.r.sup.2), while the forces that control the bead (magnetic, gravity, and inertial), are volume (.about.r.sup.3) dependent.
BCO can be reduced by employing good developer housing design and operating practices that minimize the carrier impaction rate and that avoid conditions conducive to highly de-toned carrier beads. Even so, because of stress conditions, or stringent CQ requirements (where even an occasional BCO transfer deletion may not be acceptable), it is common practice to employ a bead pick-off magnet to retrieve maverick beads.
Typically, a BCO pick-off device relies on a permanent magnet assembly with a north/south pole pair in close proximity to the p/r to create a strong field that captures the bead and moves it towards the magnet assembly. If no means is provided to remove the captured beads, over a period of time they will accumulate and rake against the p/r. To avoid this, the pick-off magnet can be enclosed in a rotating, non-magnet sleeve. The purpose of the sleeve is to prevent the beads from coming into contact with the magnet (in which case the beads are hard to remove), and to convey the captured beads away from the bead pick-off region, where they are deposited in a catch tray, or returned to the development housing sump.
The conventional bead pick-off magnet assembly has proven to be a good countermeasure for the BCO problem. However, because of their mechanical and magnetic structure, these devices cannot be made much less than about one inch in diameter, and still be effective. Unfortunately, despite the need, size rules out the use of the conventional bead pick-off assembly for many applications. For example, it may not fit in small, compact machines or in color machines that employ multiple development housings that each require a separate pick-off magnet.
The BCO problem is particularly challenging in tri-level xerography, not only because of the multiple housing requirement, but because of unique demands placed on the first and second development housings. For example, the complimentary half of the tri-level latent image is present in the first development housing, and so the qE force driving normally charged carrier beads into the complimentary image is large in magnitude compared to that of the first image development field. Hence, even moderately charged carrier beads, although repelled by the first image areas, have a tendency to deposit themselves in the complimentary image areas. The problem in the second development housing is different. Here, the need to employ "soft" or low force (magnetic) development techniques to prevent damage to the first image, means there is little, or no magnetic force available to control BCO. Furthermore, the conductive carrier (CMB) employed in tri-level xerography to avoid fringe field development, is particularly prone to inductive charging. This is especially true in the first housing because of the high reverse fields created by the complimentary latent image.
Carrier bead removal devices are known, such as for example, U.S. Pat. No. 3,894,513 to Stanley et al. and U.S. Pat. No. 3,834,804 to Bhagat et al., which use a stationary magnet having a cylindrical shell rotating thereabout to remove the ferrous carrier beads from the photoreceptor for deposit in a sump or for return to the developer housing. Other bead pick-off devices are known, such as, for example, U.S. Pat. No. 4,210,397 to Macaluso et al. which suggests the use of an electromagnetic bead collector which is periodically activated for the collection of carrier beads on a non-magnetizable surface, and de-energized to release the beads along a return path to the developer housing. However, an electromagnetic bead collector is relatively expensive, costly to implement, and requires a rather large current source.
U.S. Pat. No. 4,190,351 to Macaluso et al. shows a bead removal arrangement in which the carrier beads are removed from the photoconductive surface by means of a movable magnet and a fixed non-magnetizable shield mounted in close association between said magnet and the photoconductive surface. During the copying cycle, the magnet is moved adjacent the fixed shield to cause magnetizable articles to be drawn against the shield from the photoconductive surface. After the copying cycle, the magnet is moved away from the fixed shield to withdraw the strong magnetic field from the shield, causing the magnetizable particles to fall from the shield into a collection tray by means of gravity.
U.S. Pat. No. 4,868,607 relates to a carrier bead pick-off device for removal of carrier beads adhering to a charge retentive surface in an electrophotographic device having at least one developer housing movable into and out of developing position. A magnet is supported on the movable developer housing of the type which is movable into and out of developing position with respect to the photoreceptor. When the developer housing is moved into developing position, the magnet is correspondingly brought into a position closely adjacent to a non-magnetic carrier bead catch supported closely adjacent to the charge retentive surface. Beads are collected at the bead catch and released upon removal of the magnet from proximity to the bead catch. The non-magnetic bead catch may be supported for movement into a bead catching position when the developer housing is brought into developing position, and to a bead releasing position when the developer is removed from developing position and the magnet is removed from its position adjacent the bead catch. When the bead catch is in bead catching position, the magnet is supported so that-captured beads are collected at the bead catch by the magnetic force. When the magnet is removed from that position, there is no longer a magnetic force attracting beads to the bead catch, and the movement of the bead catch to a bead release position allows collected beads to fall to a storage location.
Magnetic arrangements are known for the removal of magnetic material from a surface, including U.S. Pat. No. 4,552,451 to Yamazaki et al. and JP-A 59-94776 to Iwamasa.
It would be highly desirable to simply provide a magnetic member closely associated with the charge retentive surface for the removal of carrier beads therefrom, avoiding the need for moving parts or complex controls to operate a bead removal arrangement. However, it will no doubt be appreciated that over time, carrier beads would accumulate at such a magnetic member, and, unless removed, could cause damage or undesired abrasion of the charge retentive surface.